import tkinter as tk

window = tk.Tk()
window.title('my window')
window.geometry('800x400')

var = tk.StringVar()  #

l = tk.Label(window, textvariable=var, bg='green', font=('Arial,12'), width=15, height=2)

l.pack()

on_hit = False


def hit_me():
    global on_hit

    if on_hit == False:
        on_hit = True
        var.set('you hit me')
    else:
        on_hit = False
        var.set('')


b = tk.Button(window, text='hit me', width=15, height=2, command=hit_me)
b.pack()

window.mainloop()
